refactor: Rename bugdown_version to markdown_version in models.py.
refactor: Rename bugdown_version to markdown_version in models.py.

This commit is part of series of commits aimed at renaming bugdown to
markdown.

andersk/zulip,synicalsyntax/zulip,eeshangarg/zulip,synicalsyntax/zulip,showell/zulip,rht/zulip,kou/zulip,zulip/zulip,rht/zulip,hackerkid/zulip,showell/zulip,synicalsyntax/zulip,eeshangarg/zulip,hackerkid/zulip,punchagan/zulip,rht/zulip,showell/zulip,punchagan/zulip,rht/zulip,punchagan/zulip,kou/zulip,brainwane/zulip,rht/zulip,synicalsyntax/zulip,punchagan/zulip,kou/zulip,andersk/zulip,punchagan/zulip,synicalsyntax/zulip,kou/zulip,brainwane/zulip,hackerkid/zulip,eeshangarg/zulip,zulip/zulip,eeshangarg/zulip,zulip/zulip,synicalsyntax/zulip,showell/zulip,punchagan/zulip,showell/zulip,brainwane/zulip,showell/zulip,andersk/zulip,hackerkid/zulip,brainwane/zulip,eeshangarg/zulip,hackerkid/zulip,eeshangarg/zulip,andersk/zulip,brainwane/zulip,kou/zulip,andersk/zulip,showell/zulip,synicalsyntax/zulip,zulip/zulip,andersk/zulip,zulip/zulip,kou/zulip,hackerkid/zulip,punchagan/zulip,kou/zulip,zulip/zulip,hackerkid/zulip,rht/zulip,eeshangarg/zulip,brainwane/zulip,brainwane/zulip,rht/zulip,andersk/zulip,zulip/zulip